Microchannel plates (MCP) have long been used in a variety of different applications. As shown in FIG. 1 from J.Wiza, "Microchannel Plate Detectors", Nucl. Instr. Methods 162, 587 (1979),and as their name suggests, an MCP 10 is a plate of material with extremely thin holes or channels 12 running from one side of the plate to the other. The actual channels formed in the MCP can be seen in photos taken from scanning electron microscopes. For example, FIG. 2 is an electron micrograph of the top plan view of a typical MCP and is taken from M.A. Barstow et al., NIM A286 350 (1990).
The walls of the channels are coated with a secondary emitter, which means that whenever an electron collides with the channel's walls, at least one other electron is produced in response to the collision. Each channel of an MCP is generally cylindrical and has a diameter and length. The ratio of diameter to length is known as the channel's aspect ratio, and is represented by the following formula: EQU .alpha.=L/d, (1)
where L is the length of the channel (the thickness of the plate) and d is the diameter. The actual sizes of the channels can depend on the material used to form the MCP. For example, channels in glass-based MCP's are typically 10-15 .mu.m in diameter, but can range anywhere from about 5 .mu.m to 100 .mu.m in diameter. The aspect ratio a of glass-based MCP's is typically between 40 and 100.
In normal commercial applications, the plate is made of a material which functions as both a secondary emitter and an insulator (i.e. it prevents electrons from flowing through the material). For example, matrix 15 may consist of lead glass which has been changed by reduction in hydrogen to form a secondary emitter surface consisting of Pb/PbO. In some instances, the glass is covered by a tin layer of SiO.sub.2 to further improve secondary electron yield. The lead glass matrix may be formed by repeatedly drawing a preform of etchable glass fibers clad with the lead glass until the fiber core shrinks to the desired diameter. The glass is then sawed into discs of the appropriate thickness and the cores etched away. The resultant disc is then heated in a reducing atmosphere to produce a Pb-rich, weakly conducting surface. A thin layer (20-200 nm) of SiO.sub.2 is then deposited over this surface to provide a surface with a high secondary yield. Finally, the top and bottoms of the disc are coated with metallic electrodes 17 and 18.
In operation, a high voltage is applied from electrode 17 to electrode 18, i.e. between the front and back surfaces of the MCP (the resistance of the matrix is typically 10.sup.9 .OMEGA.). As shown in FIG. 3, which is a schematic of a MCP, the electric field created by the electrodes accelerates the electrons 23 through the channel and the electrons collide with the channel's walls 21. Whenever an electron collides with a wall, at least one other electron is produced in response and accelerated through the channel. These new electrons also collide with the channel walls and produce even more electrons. The process continues so that the successive secondary emission creates a cascade of electrons exiting the channel. Each channel acts as a continuous dynode. Thus, an MCP consists of an array of parallel and miniature channels which function as electron multipliers.
The average number of electrons emitted for each collision may be approximated by a formula. Specifically, the secondary emission yield of electrons emitted per incident electron is: EQU .delta.=AV.sub.c.sup.1/2, (2)
where A is a proportionality constant and V.sub.c is the electron collision energy in eV. For a wide range of lead glasses used in MCP activation, A is about 0.2 and .delta..sub.max is about 3.5 at 0.3 kV.
An MCP has many different applications, but its main commercial use is for imaging amplification applications such as night vision goggles and the like. FIG. 4 schematically represents a gated MCP photomultiplier tube offered by Hamamatsu Corp for high rate applications. In operation, photons of light 42 bounce off of the object 40 to be viewed and are projected onto photocathode 44 of the tube 46. Wherever a photon hits photocathode 44, an electron 45 is generated from the material and travels under the presence of an electrical field from the photocathode and into the channels of MCP 48. Due to the secondary emission described above, the number of electrons exiting a channel will be greater than the number of electrons entering the channel. Thus, MCP 48 amplifies the photoelectron pattern of the optical image 40 being projected onto photocathode 44. The resulting amplified electron image exiting the MCP is then projected onto silicon target anode 49. The photocathode 44, MCP 48 and anode target 49 are disposed within housing 47 and kept in a vacuum. In a manner well known in the art, the anode converts the amplified electron image into a pixelated image which may be viewed on a computer-controlled display. Alternatively, the electrons may be projected through an electron lens and then directly onto a phosphor screen, which causes the phosphor to glow and form an amplified image of object 40.
MCP's are used in applications other than night vision. For electron amplification, charged particle and energetic photon detection (mainly UV or soft x-rays), an MCP has the virtues of high speed (sub-ns rise/fall times, transit time spreads less than 100 ps), high gain (typically about 10.sup.3 -10.sup.6 /MCP stage), two-dimensional incident electron image preservation under amplification, immunity to magnetic fields, and compactness. Thus, an MCP may also be used in: the fastest rise time and lowest time-jitter photomultiplier tubes; for charged particle and photon detection in a wide variety of physical science instrumentation; in streak cameras; as amplifiers for cathode ray tube beams; and, potentially, in many other vacuum electronics devices as a gain mechanism.
Despite the broad applications for MCP's, typical MCP's suffer from a number of disadvantages inherent in their manufacture. Specifically, the availability and efficiency of using an MCP in a particular application will depend upon the limitations of gain and gain degradation with accumulated charge.
The gain of a channel represents the number of electrons generated by a channel in response to electrons entering the channel. Thus, the greater the gain, the better the amplification of the image being viewed. The gain of a channel is given by: EQU G=(AV/2.alpha.V.sub.0.sup.1/2).gamma. (3)
where EQU .gamma.=4.alpha..sup.2 (V.sub.0 /V) (4)
and V is the total channel voltage and V.sub.0 is the initial energy of the secondary electron about (1 eV). Gains for lead glass-based channels are normally between 10.sup.3 and 10.sup.4 at V=1000 V, depending on the activation processing (i.e., the creation of a secondary emitter surface in the channel). As the voltage applied across the MCP increases, the number of collisions within the channel walls decreases and the secondary emission will be less orthogonal to the walls. Accordingly, the gain saturates with increasing voltage. The maximum aspect ratio and gain are given by: EQU .alpha.M=AV/(3.3V.sub.0.sup.1/2) (5)
and EQU 1n G.sub.M =0.184A.sup.2 V. (6)
For typical MCP channels using activated Pb-glass, the gain G.sub.M is about 1500 at V=1 kV with the aspect ratio .alpha..sub.M at about 60. When the aspect ratio increases beyond .alpha..sub.M (for example, the plate thickness increases while the hole diameter remains constant), the gain saturates at the maximum gain value. In other words, the maximum gain of a channel will depend on a number of factors, including keeping a set proportion between the diameter and length of the channel. In straight-channel glass MCP, the gain limit is typically between 10.sup.3 and 10.sup.5.
Another major problem for the MCP gain mechanism using reduced lead glass channels or other activation methods on glass is the decrease in gain with total charge drawn from the channel. Put simply, the MCP's tend to wear out over time especially if the MCP's are not stored in a vacuum or are exposed to high temperatures. Such high temperatures can occur if the plate's material becomes hot from ohmic heating when a voltage is applied to the electrodes. The change in gain with use is a major impediment to more widespread use of MCP, and is a major challenge for MCP manufacturers.
Degradation of gain with operation is almost unavoidable with the chemistry of silica-based glasses because of the evolution of and reaction with impurities in the channel by the electron bombardment of the channel surface. The problem is especially acute with newer, higher yield lead glass channels. The problem of gain degradation with increased use can be understood in terms of a simple surface ionization process during secondary emission that results in the removal of the secondary emitter through reaction with a finite population of poisoning species such as adsorbed gasses or glass impurities. This explanation can describe gain degradation over a wide range of cumulative area charge drawn from the channel. To the applicant's knowledge, the best gain as a function of Q (cumulative charge density) is a reported 50% gain reduction after 0.1 to 0.01 C/cm.sup.2 is drawn from the MCP and is typically closer to the lower value. The newer glass channels with the half-gain at the 0.1 C/cm.sup.2 level suffer from a dark noise which is 5 times that of the "standard" MCP. Typical gain degradations to the half-value correspond to an exposure of about 10.sup.14 electrons/cm.sup.2 incident at a gain of 1,000. Typical gain reduction with accumulated charge has been discussed by A. Authinarayanan and R. Dudding, Aadv. Electron. Physics 40A, 167 (1976). Gain reductions with accumulated charge limit the length of operation and the precision of measurements made with these detectors.
In summary, prior art MCP's suffer from a number of disadvantages. They are relatively expensive, being consistently near or above $100/cm.sup.2. They are only available in limited dimensions: While MCP's are commonly a few centimeters in diameter, they are generally not available in sizes greater than 11 cm.times.11 cm. Moreover, because it is difficult for prior art MCP's to have extremely small channel diameters and because the resolution of an amplified image is proportional to the density of the channels, prior art MCP's cannot be both miniature and have relatively very high resolutions. Prior art MCP's also suffer from steady gain degradation. In addition, prior art MCP's have spatial non-uniformities in gain, that is the gain changes from one channel to the next across the plate. Typical measurements show that the difference in gain between channels varies by about a factor of two and that there is about a 30% change in FWHM of the gain distribution across a 4 cm diameter MCP when a uniform input is applied to all channels.
Accordingly, there is a need for a lower-cost MCP which overcomes the foregoing disadvantages. Many applications would benefit from such an MCP, including imaging photodetectors and intensifiers, energetic particle calorimetry in nuclear or medium energy physics, fusion reaction products, particle imaging, medical imaging, or energetic particle track imaging using scintillating fibers.